Modeling Reaction and Transport Effects in Stereolithographic 3D Printing

Continuous stereolithography has recently emerged as a leading technology in additive manufacturing (3D printing). Though several methods for continuous printing have been reported, they all share the benefit of reducing forces on the growing part and eliminating adhesion to the resin bath due to the introduction of the dead zone, a region where polymerization does not occur. The recently developed dual-wavelength approach, in which photoinitiation and photoinhibition of polymerization are controlled via different wavelengths of light, has achieved unprecedented vertical print speeds via expansion of the dead zone. We address several limitations in dual-wavelength continuous printing (and some within continuous stereolithography more broadly) via theoretical and computational modeling and the use of spatially varying exposure patterns. First, we address the problem of cure-through, undesired curing along the axis of exposure, which is more significant in continuous stereolithography than in traditional layer-by-layer stereolithography. Recognizing that the use of highly absorbing resins to improve layer resolution inherently limits achievable print speeds, we developed a method to improve part fidelity in low- to moderate-absorbance resins through modification of the images projected during printing. We derive a mathematical model to describe dose accumulation during continuous printing, describe the resulting grayscale-based correction method, and experimentally verify correction performance. Using optimized parameters with a high absorbance height resin (2000 um), feature height errors are reduced by over 85% in a test model while maintaining a high print speed (750 mm/h). Recognizing the limitations of this model, we developed a kinetics-based curing model for dual-wavelength photoinitiation/photoinhibition under variable intensities. The model is verified via experimental characterization of two custom resins using cured height and dead zone height experiments. For the two custom resins characterized, the model achieves R2 values of 0.985 and 0.958 for fitting uninhibited cure height data and values of 0.902 and 0.980 for fitting photoinhibited dead zone height data. The model is also applicable to resins in standard layer-by-layer stereolithography, and for commercial resin cure height data, our model performs similarly to the standard Jacobs model, with all R2 values above 0.98. Finally, we introduce the complexities of resin flow during continuous printing. The kinetic curing model is used in a computational fluid dynamics model to analyze dead zone uniformity, which we find is greatly affected by exposure intensity ratio, while print speed and part radius have minor effects. We find that relatively small variations in the intensity ratio (25%) can have large effects, going from good printing conditions to print failure (curing to the window) or to significant nonuniformity (maximum dead zone height over three times the minimum). We optimize exposure conditions to maximize dead zone uniformity, finding that the ability to pattern light sources is critical in generating uniform dead zones: for a 10 mm radius cylinder, over 90% of the dead zone is near the optimized value when using patterned intensity functions, compared with only 18% when using constant intensity values. In printing experiments, we find that an optimized intensity function can, without modification, successfully produce difficult-to-print parts. Taken as a whole, the work advances our understanding of the dual-wavelength approach in continuous stereolithography, improves printing performance, and motivates future research into the wide range of physical phenomena affecting the system.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163239/1/zdpritch_1.pd

Proof-of-concept investigation of Active Velcro for smart attachment mechanisms

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76230/1/AIAA-2001-1503-863.pd

Low cost fabrication processing for microwave and millimetre-wave passive components

Microwave and millimetre-wave technology has enabled many commercial applications to play a key role in the development of wireless communication. When dissipative attenuation is a critical factor, metal-pipe waveguides are essential in the development of microwave and millimetre-wave systems. However, their cost and weight may represent a limitation for their application. In the first part of this work two 3D printing technologies and electroless plating were employed to fabricate metal pipe rectangular waveguides in X and W-band. The performance for the fabricated waveguides was comparable to the one of commercially available equivalents, showing good impedance matching and low attenuation losses. Using these technologies, a high-performance inductive iris filter in W-band and a dielectric flap phase shifter in X-band were fabricated. Eventually the design and fabrication of a phased antenna array is reported. For microwave and millimetre-wave applications, system-on-substrate technology can be considered a very valuable alternative, where bulky coax and waveguide interconnects are replaced by low-loss transmission lines embedded into a multilayer substrate, which can include a wide range of components and subsystems. In the second part of this work the integration of RF MEMS with LTCC fabrication process is investigated. Three approaches to the manufacture of suspended structures were considered, based on laser micromachining, laser bending of aluminium foil and hybrid thick/thin film technology. Although the fabrication process posed many challenges, resulting in very poor yield, two of the solution investigated showed potential for the fabrication of low-cost RF MEMS fully integrated in LTCC technology. With the experience gained with laser machining, the rapid prototyping of high aspect ratio beams for silicon MEMS was also investigated. In the third part of this work, a statistical study based on the Taguchi design of experiment and analysis of variance was undertaken. The results show a performance comparable with standard cleanroom processing, but at a fraction of the processing costs and greater design flexibility, due to the lack of need for masks.Open Acces

Additive manufacturing (3D print) of air-coupled diaphragm ultrasonic transdrucers

Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency.Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency

3D Printed Microfluidic Devices

3D printing has revolutionized the microfabrication prototyping workflow over the past few years. With the recent improvements in 3D printing technologies, highly complex microfluidic devices can be fabricated via single-step, rapid, and cost-effective protocols as a promising alternative to the time consuming, costly and sophisticated traditional cleanroom fabrication. Microfluidic devices have enabled a wide range of biochemical and clinical applications, such as cancer screening, micro-physiological system engineering, high-throughput drug testing, and point-of-care diagnostics. Using 3D printing fabrication technologies, alteration of the design features is significantly easier than traditional fabrication, enabling agile iterative design and facilitating rapid prototyping. This can make microfluidic technology more accessible to researchers in various fields and accelerates innovation in the field of microfluidics. Accordingly, this Special Issue seeks to showcase research papers, short communications, and review articles that focus on novel methodological developments in 3D printing and its use for various biochemical and biomedical applications

Investigation of isocyanate-based dual-cure resins and their suitability for additive manufacturing

The development of materials for vat photopolymerization printing technologies has gained increasing attention within the last ten years. To date, the choice of materials for this printing technology is limited to photopolymers that tend to result in highly crosslinked, but usually brittle material properties. In this work, a dual-cure approach was developed, with the target to improve the overall material properties of 3D printed objects. For this purpose, the orthogonal and sequential polymerization of acrylate- and polyisocyanate based precursors was investigated. Photosensitive polyurethane acrylates were synthesized and mixed with low viscous polyisocyanates. The sequential polymerization was triggered by UV-light and heat, respectively. The reaction of the isocyanate groups in the heat, resulted in a polyurea network, confirmed by infrared spectroscopy. The morphology of resulting combined networks was systematically investigated by dynamic mechanical analysis and atomic force microscopy and showed that phase-separated morphologies with two distinct glass transition temperatures were formed, leading to a 10 fold increase in toughness in comparison to the neat polymeric networks. First printing trials showed the feasibility of the resin system for vat photopolymerization. The approach was extended to the sequential polymerization of isocyanate terminated prepolymers and low viscous acrylates. Varying the crosslinking densities of each polymeric network resulted in different morphologies as well as thermomechanical properties. Typical elastomers were obtained by a low crosslinking density of both polymeric networks. Cyclic loading-unloading measurements on 3D printed specimen showed the energy elastic behavior of the materials and an elongation at break of 101 % and a tensile strength of 3.4 N·mm-2 were achieved. A dual-cure resin based on acrylate-functionalized polyisocyanates was evaluated and resulted in thermally- and chemically stable 3D printed materials with the option to generate adoptable and catalytically active surfaces. The suitability of this resin to 3D print chemical reaction ware was demonstrated. In summary, the results obtained in this work show the tuneability and variability of combining acrylate- and polyisocyanate precursors, contributing to the increasing demand of new and improved materials for vat photopolymerization